It must be emphasized, the concentrations of radon-222 and radon-220 in the soil and in the building materials vary over many orders of magnitude from place to place and show significant time variation at any given site. Locations with higher radon background are well mapped in each country. In the open air, it ranges from 1 to 100 Bq/m3, even less (0.1 Bq/m3) above the ocean. In caves or aerated mines, or ill-aerated houses, its concentration climbs to 20–2,000 Bq/m3. In the outdoor atmosphere, there is also some advection caused by wind and changes in barometric pressure.
Problems with radon are in houses, where it can accumulate especially, due to its high density, in low areas such as basements and crawl spaces. Radon can also occur in ground water – for example, in some spring waters and hot springs. Several possibilities exist for the release of radon into houses. The fact radon is a noble gas plays a crucial role in spreading of all its daughter nuclei. Simply radon is a transport medium from bedrock to atmosphere (or inside buildings) for its short-lived decay products (Pb-210 and Po-210), that posses much more health risks. The main sources are the rock or soil on which the house is built, as well as the water supply. The main mechanism for the entry of radon into buildings is diffusion through the soil. As a gas, radon diffuses through rocks and the soil. The radon gas can penetrate into the house through cracks (due to a chimney effect) in the floor and walls of the basement. Heating of the air creates a suction of air from the lower part of the house, towards the higher part of the house. Without any radon membrane, this actually means that air from the ground beneath the house is sucked into the house through numerous floor cracks and openings.
Furthermore, the building materials (e.g., some granites) is also a source for radon. Another source for radon is the water supply. Water from wells, in particular in regions with radium rich granite, may contain high radon concentrations. This is material with higher uranium/radium concentrations from which radon is continuously generated. Such materials, e.g., slag, fly ash, etc., could be used in some locations. For building materials that are used for the construction of houses, the critical limits for the specific radium concentrations must be determined.
The greatest risk of radon exposure arises in buildings that are airtight, insufficiently ventilated, and have foundation leaks that allow air from the soil into basements and dwelling rooms. The inside radon level vary considerable with weather, time of the year and even time of the day – and of course with the airing system. For example, sleeping with an open window can reduce the radon content considerable.
Most countries have adopted a radon concentration of 200–400 Bq/m3 for indoor air as an Action or Reference Level. If testing shows levels less than 4 picocuries radon per liter of air (150 Bq/m3), then no action is necessary. Very high radon concentrations (>1000 Bq/m3) have been found in houses built on soils with a high uranium content and/or high permeability of the ground.
Mitigation of radon in the air is accomplished through ventilation, either collected below a concrete floor slab or a membrane on the ground, or by increasing the air changes per hour in the building. A radon resisting membranes are usually produced from low density polyethylene (LDPE) and are extended across the whole of the building, including the floor and walls. Another way for mitigation of radon is a treatment system using aeration or activated charcoal to remove radon from domestic water supplies.
See also: Radon – Health Effects